This invention relates generally to oscillators and more particularly to voltage controlled oscillators (VCO).
The generation of a sine wave of a given frequency is an important building block upon which many circuits are based. Examples of these circuits include radio receivers and transmitters employing mixers to translate the frequency of an information-bearing signal. There are two basic approaches to the design of sine wave generators. The first is to design a non-linear oscillator, which generates square or triangular waveforms, and apply the waveform to a sine wave-shaper, which usually consists of diodes and resistors. These non-linear oscillators are also referred to as function generators. The second approach employs a positive-feedback loop that contains a frequency-selective network. The loop is designed to have a gain of unity at a single frequency determinedly the frequency selective network. In this type of oscillator, called a "linear oscillator," sine waves are generated essentially by a resonance phenomenon. This application deals with this latter approach.
An example of a linear oscillator is shown in FIG. 1A. The oscillator, shown generally at 10, includes a bipolar junction transistor (BJT) Q1 biased by a current source 12 and loaded by a load resistor R.sub.O. The oscillator includes a delay line 14 interposed between the load resistor and the collector of transistor Q1. The base of Q1 is then connected to an output terminal 16 thereby forming a loop with the delay line 14 and the collector to base junction of the Q1. The delay line 14 has a time delay or, alternatively, a wavelength, which is selected to correspond to one-half the period of oscillation of a sinusoidal voltage V.sub.O produced on output terminal 16. The other one-half of the period is produced by inverter-connected transistor Q1, thus satisfying the oscillator criteria, i.e., unity gain and 360 degrees of phase shift.
Although discrete delay lines can be used as shown above, a more common approach is to use a L-C tank circuit such as shown in FIG. 2 to produce the delay. Oscillator 30 of FIG. 2 includes two inductors L.sub.1 and L.sub.2 and two varactors CR1 and CR2, though fixed capacitors can also be used in place of varactors if tuning is not required. The two L's and two C's form a single tank circuit having an equivalent inductance and equivalent capacitance. Varactors CR1 and CR2 are used in the tank circuit so that the frequency of oscillation can be modulated by a tuning voltage V.sub.TUNE applied to control terminal 32. The variable capacitance of the varactors provides some fine-tuning of the oscillator frequency. The oscillator 30 is commonly referred to as a voltage controlled oscillator (VCO).
One limitation of the oscillator 30 is that the frequency of oscillation can only be tuned over a very small range because of the limitations of the varactors. The oscillator 40 shown in FIG. 3 overcomes the limitation of the tuning range of oscillator 30 by including multiple varactors CR1-CR3 that are switched into the oscillator circuit responsive to respective control currents I.sub.SW1 -I.sub.SW3. Each varactor CR1-CR3 has a different nominal capacitance so as to provide a unique base frequency for the oscillator when that particular varactor is enabled by the corresponding control current. The varactors, therefore, each provide a unique frequency band of oscillation. The frequency of oscillation can then be tuned within that band by modulating the tuning voltage V.sub.TUNE applied to a tuning input 42. With proper selection of the varactors, the oscillator 40 can provide a substantially continuous frequency range that greatly exceed that of oscillator 30 shown in FIG. 2. Conversely, a single varactor can be made to work in concert with a multiplicity of inductors to achieve the same result.
A characteristic of all of these oscillators is that their phase noise is only as good as the Q of the components that comprise the delay elements, i.e., the inductors and varactors. This presents a particular problem for integrated or monolithic circuits because it is difficult to achieve high Q inductors and varactors. For example, a typical on-chip inductor has a Q of approximately two to three, but a Q of approximately 50 is typically needed for adequate phase noise. Varactors have better Q's than inductors when formed on-chip, but are still inadequate for low phase noise.
An obvious solution is to provide these components in their discreet form and place them off-chip. There are several drawbacks to this approach. First is cost. The cost of providing additional external components adds to the expense of manufacturing the oscillator, not only in terms of direct materials costs, but also in terms of support and service, because of the added failures due to the manufacturing and placement of these additional parts. These additional components also consume precious real estate on a printed circuit board which may increase the form factor of the resultant product.
There are also mechanisms that compromise the Q of these off-chip solutions as well. The first is the interconnect bond wire resistance, which is a major contributor. Second is the external parasitics between the chip package and the external component. For the band switchable oscillator shown in FIG. 3, the resistance of switches (CR4, CR5, CR6) used to select the varactors appear in series with the varactors and therefore compromise the Q of those elements. For these reasons, a need remains for a fully integrated variable frequency oscillator with low phase noise.